The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method and apparatus for handling multiple resonance frequencies in disc drives using active damping.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so the data can be successfully retrieved from and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and accepting data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the transducer head away from the disc. At the same time, the air rushing past the cavity or depression in the ABS produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider that is directed toward the disc surface. The various forces equilibrate so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically equal to the thickness of the air lubrication film. This film eliminates the friction and the resulting wear that would occur if the transducing head and the disc were to be in mechanical contact during the disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on the storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data from a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. Some disc drives have a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during read or write operations using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base of the disc drive, and may also be attached to the top cover of the disc drive. A yoke is attached to the actuator. A voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor (VCM) used to rotate the actuator and the attached transducer(s). A permanent magnet is attached to the base and the cover of the disc drive. The VCM which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. The yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and the yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive the voice coil so as to position the transducer(s) at a target track.
Two of the ever constant goals of disc drive designers are to increase the data storage capacity of disc drives, and to decrease the amount of time needed to access the data. To increase storage capacity, current disc drives have increased numbers of tracks per inch (TPI). Put simply, current disc drives squeeze more tracks onto the same size disc. Decreasing the amount of time needed to access the data can be thought of as increasing the speed at which data is retrieved. Increasing the speed at which data is retrieved is very desirable. Any decreases in access time increase the speed at which a computer can perform operations on data. When a computer system is commanded to perform an operation on data that must be retrieved from disc, the time needed to retrieve the data from the disc is often the bottleneck in the operation. When data is accessed from a disc more quickly, more transactions can generally be handled by the computer in a particular unit of time.
A rotating disc data storage device uses a servo system to perform two basic operations: track seeking and track following. Track seeking refers to the ability of the disc drive and the servo system to move the read/write transducer head of the disc drive from an initial track to a target track from which data is to be read, or to which data is to be written. The settling of the transducer head at the target track is referred to as seek settling. Track following, which is performed after the head has been aligned with a target track, refers to the ability of the disc drive and the servo system to maintain the read/write head positioned over the target track. Note that, to effectively perform track seeking and track following in a disc drive with increased TPI, the servo open loop bandwidth of the system must also be pushed or increased.
Structural resonance in disc drives is one of the major challenges faced by disc drive designers in general, and disc drive servo control designers in particular. The structural resonance, such as arm bending mode resonance and coil bending mode resonance, will introduce problems in the operation of a disc drive""s VCM during seek settling and even during track following. Due to structural resonance, the position error signal (PES) for the actuator arm of a disc drive will oscillate during seek settling and track following, thus adversely affecting the settling time and the drive performance of the disc drive. The effects of structural resonance are getting worse with yearly increases in the number of TPI and in the servo bandwidth of the disc drives. As the number of TPI increases, the tracks become thinner and therefore it becomes crucial for the disc drives to minimize or eliminate resonance which can cause the actuator arm to swing to off-track positions when the actuator resonates during seek settling or track following. As the servo bandwidth increases, the susceptibility of the actuator to vibrations induced at the actuator""s resonant frequency increases, which may result in greater off-track disturbances of the heads.
While a certain amount of resonance is acceptable, the acceptable amount of resonance decreases as the number of TPI, and the servo bandwidth, increase. If the resonance is reduced or eliminated, the number of missed revolutions of the disc will be minimized and the access times will decrease. Reducing resonance will also help improve a disc drive""s through-put performance. If resonances in the actuator arm at frequencies associated with normal operation of the disc drive are reduced or eliminated, seek settling and track following will also be improved since the servo system will not be attempting to counter the effects of a resonating arm swinging across a desired track from an off-track position on one side to an off-track position on the other side of the desired track during the track settling or track following.
A common approach for addressing the structural resonance problem is to include an analog or digital notch filter to attenuate the resonant modes at particular frequencies. The control signal from the servo controller is passed through the notch filter before driving the VCM. Unfortunately, due to the nature of notch filters, the notch-filter approach introduces a large phase lag around the notch center frequency. Thus, while this approach is often used in disc drives when the resonance frequency is high compared to the servo bandwidth, the notch-filter approach is not applicable for handling resonance frequencies in disc drives that are near the servo open loop gain crossover frequency since the notch filter would cause an unacceptable phase margin drop. For example, the resonance frequencies of the arm bending mode and the coil bending mode resonance are about 700 Hz and 1000 Hz, respectively, which are both near the servo open loop gain crossover frequency. Thus, these structural resonance modes can cause problems in the track seeking and following operations which are not adequately addressed by the notch-filter approach. The seek settling and track following problems are worse if these frequencies coincide. Therefore, the notch-filter approach alone is not useful or adequate for handling certain resonance modes.
Therefore, what is needed is an improved method and apparatus for handling resonance effects in disc drives. There is also a need for a method and apparatus for handling resonance effects in disc drives which improves seek settling and/or track following in the disc drives, and may be used with disc drives that have increased numbers of TPI and increased servo bandwidths. There is also a need for a method and apparatus for handling structural resonance effects in disc drives that have resonance frequencies at or near the servo open loop gain crossover frequency of the disc drives. There is also a need for a method and apparatus for handling both high and low resonance frequencies in disc drives without substantial performance losses.
In practice, some disc drives experience problematic resonance effects at multiple resonance frequencies during seek settling and even during track following. These resonance frequencies may often appear after a short seek. One or more of these resonance frequencies may be high compared to the servo bandwidth of the disc drive, while another one or more of these resonance frequencies may be at or near the servo bandwidth. For example, a disc drive may experience a first resonant frequency of about 700 Hz during seek settling due to coil bending mode resonance, and may also experience a second resonant frequency of about 1000 Hz during seek settling due to the arm bending mode resonance. Both of these resonant frequencies are typically at or near the servo open loop gain crossover frequency of the disc drive. Thus, the use of multiple notch filters to handle these multiple resonance frequencies will not be applicable. Therefore, there is also a need for a method and an apparatus for effectively handling multiple resonance frequencies in disc drives.
The present invention relates to a method and apparatus which use active damping for handling multiple resonance frequencies in disc drives. This active damping approach advantageously handles resonance effects which occur at high and/or low frequencies without adverse affect on the performance of the disc drives. For example, this active damping approach can handle multiple resonant frequencies including at least one resonant frequency existing at or near the servo open loop gain crossover frequency of the disc drives. Thus, for example, the arm bending mode resonance and the coil bending mode resonance can both be effectively handled. Other combinations of multiple resonant frequencies can also be efficiently handled.
In accordance with one embodiment of the invention, a method of handling multiple resonance frequencies in a disc drive includes the steps of monitoring a position error signal for an actuator arm of a disc drive, generating a plurality of feedforward compensation signals from the position error signal using a plurality of bandpass filters, and applying the compensation signals to a servo control signal. Each filter has a center frequency that is set to a problematic resonance frequency.
In accordance with another embodiment of the invention, a method of handling multiple resonance frequencies in a disc drive includes steps of monitoring a position error signal for an actuator arm of a disc drive, generating a plurality of feedforward compensation signals from the position error signal using a plurality of bandpass filters, and applying the compensation signals to a servo control signal. Each filter has a center frequency set to a problematic resonance frequency, and the method also includes identifying the problematic resonance frequencies of the drive.
In one embodiment, identifying the problematic resonance frequencies of the disc drive includes the steps of commanding a movement of the actuator arm of the disc drive, collecting data points for the position error signal for the arm that are associated with the movement, and performing a digital fourier transform (DFT) of the collected position error signal data points to identify actual resonant frequencies.
In another embodiment, identifying the problematic resonance frequencies of the disc drive includes the steps of commanding a movement of the actuator arm, collecting zero-crossings data for the position error signal that are associated with the movement, and analyzing that data to identify actual resonant frequencies.
In another embodiment, identifying the problematic resonance frequencies of the disc drive includes the steps of defining a resonance frequency list including a plurality of possible problematic resonance frequencies that may appear during operation, and performing a principal components analysis to identify problematic resonance frequencies included in the resonance frequency list that actually appear.
In another embodiment, identifying the problematic resonance frequencies of the disc drive includes the steps of defining a resonance frequency list including a plurality of possible problematic resonance frequencies that may appear during operation and, for each possible problematic resonance frequency in the list, measuring a first settling time without compensation for the resonance frequency, measuring a second settling time with compensation for the resonance frequency, taking the difference between the first and second settling times, and determining if the resonance frequency is non-problematic by comparing the magnitude of the difference between the two settling times to a threshold value. The identifying also includes performing a principal components analysis of the possible problematic resonance frequencies that remain after determining the non-problematic resonance frequencies to identify the problematic resonance frequencies that actually appear.
In another embodiment, identifying the problematic resonance frequencies of the disc drive includes the steps of defining a resonance frequency list including a plurality of possible problematic resonance frequencies that may appear during operation, commanding a booster to boost the servo control signal at one of the possible problematic resonance frequencies in the list to generate a boosted control signal which is provided to an assembly for actuating the arm, and monitoring the position error signal to determine if the frequency at which the servo control signal was boosted by the booster is problematic.
In accordance with another embodiment of the invention, an apparatus for handling multiple resonance frequencies in a disc drive includes means for monitoring a position error signal for an actuator arm of the disc drive, means for generating a plurality of feedforward compensation signals from the position error signal using a plurality of bandpass filters, each of the bandpass filters having a center frequency that is set to a problematic resonance frequency, means for applying the plurality of feedforward compensation signals to a servo control signal, and means for identifying the problematic resonance frequencies of the disc drive.
These and various other features as well as advantages which characterize the present invention will be apparent to a person of ordinary skill in the art upon reading the following detailed description and reviewing the associated drawings.